Compact Muon Solenoid (CMS)
2013-04-22 07:41 UTC |
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The Compact
Muon Solenoid (CMS) is located in an underground detector hall a
hundred metres below the village of Cessy, France, almost directly
across the diameter of the LHC from CERN headquarters and the ATLAS
detector. Even before you walk into the building, it's apparent that
CMS is “compact” only in a relative sense—everything
you see is on a gargantuan scale.
Prof. Karsten Eggert of the TOTEM experiment gave us an introductory
briefing in the CMS conference room, describing the LHC, CMS, TOTEM,
and the science performed by the detectors we were about to see.
Since the LHC and CMS are in a two year shutdown for maintenance
and upgrades, the CMS control room was a sea of blank screens.
Hats on! We wait for the elevator to take us down into the
detector cavern. The cavern is 100 metres below the surface,
so the elevator ride takes a while. This is one of the few
places on Earth where people are counseled that in case of fire
or release of nitrogen or helium (both of which are nontoxic
but will cause asphyxiation by displacing breathable oxygen)
to take the elevator to escape. The elevator is equipped with a
positive pressure system for such emergencies. There is an
emergency stairway, but the chance of escaping a major asphyxiation
event by climbing the height of a thirty storey building would be
nil without an auxiliary oxygen supply. Workers in the detector
halls and tunnel carry an emergency breathing pack, but it is
intended only to allow them to get to the elevator.
Going down—all aboard!
This is the shaft to the surface through which the pieces of the
detector were lowered to be assembled in the cavern.
This door leads to the accelerator tunnel. Only a few people are
allowed in there, most certainly not including us.
Having made our way from the elevator through
a series of tunnels, we're about to enter the CMS detector hall.
The detector is surrounded by racks of electronics which perform the
first-level signal processing and event filtering
The detector is in two pieces which can be separated, as presently,
for maintenance. The beam pipe runs through the centre. Note the
people on the work platform for scale. When assembled for operation
the detector is 25 metres long, 15 metres in diameter, and weighs
around 12,500 tonnes. The magnetic field of the superconducting
solenoid magnet is 3.8 Tesla.
The right side contains an electron calorimeter,
preshower detectors, half of the hadron calorimeter, muon
tracking chambers, and the forward calorimeter.
The left side of the detector includes the interaction zone with its
barrels of close-in silicon trackers, and detectors of the same kinds
as in the right section to cover that part of the solid angle around
the interaction zone. The left face of the detector is mostly occupied
by muon chambers.
This door provides access to the detector hall. Note the retina scanner
to the right, inside the double doors. When I visited the LEP in 1996,
a system of tags, like those in mines, was used to control access and
determine who was underground. People would sometimes lend their tags
to others; the retina scanner puts an end to that (aside from James
Bond scenarios, which somehow don't seem all that out of place in
surroundings such as these).
Outside the detector hall racks and racks of electronics process
signals in real time.
Silicon detectors like this detect particles which diverge from the
point of collision. This is one petal of the
“tracker end-cap”, which forms the circular ends of
the cylindrical trackers closest to the interaction point.
These silicon “pixel” detectors run parallel to the
beam pipe and are the first to detect particles created in a collision.
Each detector segment alternates between detectors and
electronics, with the other side having electronics and
detectors offset so that most space is covered by a
detector. I didn't ask about the radiation hardening requirements
for solid state electronics to operate in this environment.
This unit is part of the “tracker outer barrel”
assembly.
Since the detector is too big to comprehend with human sight from close
quarters, this model allows visitors to understand what they've just
glimpsed.
This
document [PDF] provides technical details of the CMS detector.
This massive crane was used to lower pieces of the detector from the
surface, where it was initially assembled and tested, down into the
detector hall.
Now that's a cool vanity plate!
This document is in the public domain.